This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-402315, filed Dec. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube provided with a shadow mask.
2. Description of the Related Art
In general, color cathode ray tubes that are used in color TV sets and the like comprise an envelope, which includes a substantially rectangular face panel and a funnel. The face panel has an effective portion formed of a curved surface and a skirt portion on the peripheral part of the effective portion. The funnel is bonded to the skirt portion. Formed on the inner surface of the effective portion of the face panel is a phosphor screen that is formed of three-color phosphor layers and black non-luminous layers.
Inside the face panel, a shadow mask is opposed to the phosphor screen. The shadow mask is a substantially rectangular structure that has an effective portion, which is formed having a large number of electron beam passage apertures, and a skirt portion that is formed by bending the peripheral part of the effective portion. The skirt portion, which has no electron beam passage apertures, extends substantially at right angles to the effective portion and forms a sidewall. The shadow mask is located inside a substantially rectangular mask frame in a manner such that its skirt portion is fixed to the mask frame.
An electron gun that emits three electron beams is located in a neck of the funnel. In the color cathode ray tube, the three electron beams emitted from the electron gun are deflected by means of a magnetic field that is generated by a deflection yoke on the outside of the funnel. As the phosphor screen is scanned horizontally and vertically with the aid of the shadow mask, a color image is displayed on the screen.
In the color cathode ray tube constructed in this manner, the shadow mask is one of the essential members that has a color sorting function. In order to display an image without a color drift on the phosphor screen, the electron beam passage apertures of the shadow mask and the phosphor layers corresponding thereto must be kept in specific relative positions to ensure accurate beam landing. This positional relation must be kept constant during the operation of the cathode ray tube. More specifically, the distance (q-value) between the shadow mask and the phosphor layers must always be within a fixed tolerance.
During the operation of the color cathode ray tube of the shadow-mask type, on the other hand, only ⅓ or less of all the electron beams that are emitted from the electron gun can pass through the electron beam passage apertures of the shadow mask and reach the phosphor screen. The remaining electron beams run against the effective portion of the shadow mask and, as thermal energy, heat the shadow mask. In consequence, the shadow mask undergoes thermal expansion and causes a doming phenomenon such that it bulges on the phosphor-screen side. If the doming phenomenon causes the space (q-value) between the phosphor screen and the shadow mask to exceed the tolerance, beam landing on the phosphor layers is dislocated, and the color purity falls.
Accordingly, an Invar material that has a low coefficient of thermal expansion is frequently used for the shadow mask. On the other hand, a cold-rolled steel sheet, which is relatively low-priced, is used for the mask frame. If the shadow mask and the mask frame are heated during the operation of the color cathode ray tube, in this case, the shadow mask that is firmly welded to the mask frame is pulled by the mask frame, owing to the difference in thermal expansion between the two members.
While the color cathode ray tube is being manufactured, it undergoes heating processes at high temperature, such as a process for forming the phosphor screen, an exhaust process, and the like. Thus, a more serious tensile deformation is caused by the difference in thermal expansion between the shadow mask and the mask frame.
To lessen this effect, many consumer color cathode ray tubes are constructed so that the shadow mask is fixed to the inner peripheral side of the mask frame, as mentioned before. If the mask frame pulls the skirt portion on account of the difference in thermal expansion, in this case, that part of the skirt portion (corresponding to the substantial skirt length) which ranges from weld spots between the skirt portion and the mask frame to the upper side of the skirt portion is deformed diagonally inward. Thus, the skirt portion serves as a cushion to prevent the tensile deformation from influencing the curved surface of the effective portion of the shadow mask.
If the major and minor axes of the shadow mask become longer, as in modern large-screen color cathode ray tubes, however, the difference in elongation that is attributable to the difference in thermal expansion inevitably becomes greater. Since the tensile force that is produced by the thermal expansion of the mask frame increases in proportion to this, more serious problems are caused.
In a color cathode ray tube with a flat screen, moreover, the shadow mask is flattened to match the face panel, so that its curved surface retention strength is lower than that of a conventional shadow mask with a greater curvature. Accordingly, in this case, there is a high possibility of only a small tensile stress causing a substantial deformation of the curved surface. Thus, the tensile stress that acts on the shadow mask is expected to be minimized.
In order to restrain the tensile deformation of the shadow mask that is attributable to the difference in thermal expansion, it is desirable that the skirt portion of the shadow mask should be as deformable as possible. Thus, the tension restraining effect can be improved by maximizing the substantial skirt length. If the substantial skirt length is greater, however, then the skirt portion will more easily undergo deformation in a direction in parallel to its planes. In other words, the greater substantial skirt length implies that the shadow mask is liable to be deformed in directions perpendicular to the tube axis in the mask frame.
If the shadow mask is subjected to impact or vibration in a direction perpendicular to the tube axis, therefore, it is easily dislocated in the same direction. If this dislocation occurs, the electron beam passage apertures are dislocated to cause mis-landing such that the electron beams fail to reach the specified phosphor layers, thereby lowering the color purity. As the attachment and detachment of the shadow mask are repeated several times in a phosphor screen forming process, moreover, the shadow mask is inevitably caused to move horizontally or vertically from its set position by impact from the mask frame itself or the like. This dislocation makes it impossible to form a phosphor screen accurately, and possibly results in a serious failure. Recently, in particular, there has been an increasing demand for higher image quality levels, and the pitches of phosphor screens have been narrowed to improve their resolution. Thus, the allowance for beam landing errors is so low that more accurate beam landing is required.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube in which dislocation of a shadow mask attributable to thermal expansion can be restrained so that the production stability and image quality level are improved.
A color cathode ray tube according to an aspect of the invention comprises: an envelope including a substantially rectangular face panel, a funnel connected to the face panel, and a neck connected to the funnel; a phosphor screen formed on an inner surface of the face panel; an electron gun located in the neck and configured to emit electron beams toward the phosphor screen; a substantially rectangular shadow mask located opposite to the phosphor screen in the envelope and having a given coefficient of thermal expansion; and a substantially rectangular mask frame having a coefficient of thermal expansion different from that of the shadow mask and supporting the peripheral edge portion of the shadow mask. The face panel, shadow mask, and mask frame have a major axis extending at right angles to a tube axis and a minor axis extending at right angles to the tube axis and the major axis. The shadow mask includes a substantially rectangular effective region formed having a number of electron beam passage apertures and opposed to the phosphor screen, and a substantially rectangular skirt portion provided around the effective region and extending in the direction of the tube axis. The skirt portion has a pair of long sides extending substantially parallel to the major axis and a pair of short sides extending substantially parallel to the minor axis, the long and short sides being welded to the inner surface of the mask frame. A distance a from an extending end of the skirt portion on each long side to a weld spot is longer than a distance b from an extending end of the skirt portion on each short side to a weld spot.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.